1. Field of the Invention
The present invention relates to an external apparatus and an imaging apparatus that can correct a shift in a focus position caused by inserting or removing an optical filter.
2. Description of the Related Art
Conventionally, a camera uses an optical filter such as an infrared cut-off filter (hereinafter referred to as an “IRCF”) to block light of predetermined wavelengths in the light incident on the camera through a lens. For example, the light from an object goes through the IRCF, and this can prevent an image from becoming reddish, so that the image is similar to that viewed with the human eyes.
Further, the following camera is known. To enable the capturing of a bright image even under a low-illuminance environment, an IRCF mounted on a camera can be removed from the optical path of an imaging optical system to introduce also light of wavelengths in the infrared range into a sensor.
Generally, such a function is termed a “day/night function”. Under an environment where the object is bright, the object is captured in a day mode where the IRCF is inserted into the optical path. On the other hand, under an environment where the object is dark, the object is captured in a night mode where the IRCF is removed from the optical path.
In the state of the night mode where the IRCF is removed, light in the infrared range enters the sensor. Thus, the wavelength components of the incident light are different from those when the IRCF is inserted. Changes in the wavelengths of the light incident on the sensor may change the focus position. This may result in a defocused image.
Japanese Patent Application Laid-Open No. 2012-032689 discusses an imaging apparatus that receives wavelength information of near-infrared light through an operation unit provided in a lens system, and places on the optical axis a band-pass filter that transmits light of the received wavelength information. Further, the imaging apparatus reads from a memory an optical axis position corresponding to the received wavelength information and an optical axis position of a variable magnification lens, and locates a rear lens group at the read optical axis position, thereby preventing the influence of a focus shift.
Further, Japanese Patent Application Laid-Open No. 2006-162757 discusses a television camera that calculates correction data from the thicknesses and the refractive indices of an IRCF and clear glass, and displaces a tracking lens using the calculated correction data. The correction data is used to correct the change in the optical length due to the difference between the thicknesses of the IRCF and the clear glass. The IRCF is placed on the optical axis in a normal photographing mode, and the clear glass is placed on the optical axis in a near-infrared photographing mode.
In the conventional art discussed in Japanese Patent Application Laid-Open Nos. 2012-032689 and 2006-162757, however, the amount of correction of the focus position is determined within the camera. Thus, a camera without the function of determining such an amount of correction cannot correct the focus position. Further, a target light source may not be assumed in advance by a camera.